1. Field of the Invention
The present invention relates to a wireless transmission apparatus and a wireless transmission method for transmitting various types of information using, for example, a wireless signal, among a plurality of communication terminals.
2. Description of the Related Art
As a conventional asynchronous wireless transmission method, a wireless transmission method in which information transmitted from a high-order layer is buffered and this information is transmitted when an access control right can be acquired within a predetermined time has been generally used.
Furthermore, in the conventional asynchronous wireless transmission method, since information transmitted from a high-order layer need only be transmitted wirelessly in a short time, when a request for transmitting new information is received continuously from the high-order layer, a control method in which the new information is discarded has been generally used.
That is, if acknowledgement (ACK) information is not returned from the information receiving destination, a control method in which an error display is instantly made and the fact that transmission is impossible is returned to the high-order layer has been used. Furthermore, when an error occurs, a control method in which operation such as retransmission is performed according to the determination by the high-order layer has been conceived.
As a conventional flow control method using a sequence window, a window control method in which information concerning the size of a buffer area present in the receiving destination is first reported to the information transmission destination, and the information transmission source refrains from transmitting information exceeding this size has been used.
Furthermore, in a case where certain information is to be transmitted wirelessly, a method in which that information is divided into packets with a predetermined fragment size and the packets are transmitted has been generally used. This method of transmitting packets is widely used in wireless transmission lines.
In addition, recently, a method has been conceived in which, at the information transmission source, the information to be transmitted is divided into packets of a predetermined size, these packets are transmitted wirelessly, these packets are collected, and then the original information is restored in the information receiving destination.
This method has been considered for use in combination with a selection-repeat-resend (SR)-type automatic resend request (ARG) method, in which information on packets which were successfully received by the information receiving destination is transmitted as acknowledgement information to the information transmission source and in which only the packet which has not been received is selected and retransmitted from the information transmission source.
When the conventional selection-repeat-resend-type automatic resend request method is used, by returning the acknowledgement information collectively to a certain degree, retransmission control in packet units can be easily performed. Therefore, by decreasing the number of packets indicating the amount of information flowing through a wireless transmission line, the information traffic during retransmission can be decreased.
As a result, information traffic during retransmission can be reduced as a retransmission control method of packet transmission, and transmission suitable for a wireless transmission line having a high transmission error rate can be performed.
Such a conventional wireless transmission method is designed on the premise that, after information is transmitted, the reception is acknowledged immediately by the information receiving destination. Therefore, there is a problem in that, when asynchronous information having different destinations is transmitted wirelessly in a case where a plurality of devices is connected to a wireless transmission apparatus, each transmission must be handled by a different process.
In a conventional transmission sequence, there is a problem in that the transmission of the next asynchronous information cannot be performed until the transmission of one piece of asynchronous information which is received from the high-order layer is completed. As a result, in the transmission apparatus of the information transmission source, there is the problem of having to provide a redundant buffer area for storing data which cannot be sent to make that information effective in order to be able to deal with a case in which an asynchronous information request occurs continuously from the high-order layer.
Furthermore, in the method for performing wireless transmission to a plurality of destinations by using a conventional method, regarding the setting of various types of parameters, such as the management of a sequence number for performing retransmission control, there is also the problem of having to perform the setting individually according to the number of destinations.
Furthermore, the conventional transmission flow control method using a window is a control method which is mainly suitable for wired transmission lines in which a two-way link is reliably established and is suitable for line-switched-type information transmission lines, and there is a problem in that it is difficult to use the method for wireless transmission lines in which instability of the two-way link is permitted and for packet-switched-type wireless transmission lines. This problem occurs because no method for reliably transmitting the size of the buffer area present in the receiving destination device to a device which is an information transmission source has been provided.
Furthermore, in a case where the conventional selection-repeat-resend-type automatic resend request method is used, there is a problem in that, since the information transmission source determines whether the packet is information which is not received on the basis of the acknowledgement (ACK) information returned from the information receiving destination, a memory space corresponding to a bit map of a series of sequence numbers is required to identify which sequence number corresponds to a packet which has been transmitted and for which the acknowledgement (ACK) information has not been received.
In addition, in a case where sequence numbers are managed individually for each destination as in the conventional method, a different parameter for each receiving destination station must be provided according to the number of stations for which connection is assumed, and there is a problem in that a large memory area is required to manage these sequences.